The present invention relates in general to a clinical testing apparatus. More particularly, this invention pertains to a testing apparatus for making multiple sample clinical determinations of ionized calcium concentrations or other ions in body fluids. The aspects of this invention pertain primarily to some of the electrical phenomenon associated with the clinical testing apparatus of this invention.
U.S. Pat. No. 3,429,785 shows one probe arrangement for sampling ions in body fluids. One of the problems that is encountered is that the signals that are generated are extremely high impedance signals and thus it is difficult to handle these signals. Furthermore, it has been found that in existing machines that it is difficult to reject spurious electrical signals that may occur and interfere with the sampling operation. In these prior art machines it has been found that it is desirable to provide a shield which is attached to the electrode or probe. This shield may be in the form of a metallic paint or a metallic impregnated material which is fastened to the electrode. However, the presence of this shield prevents one from easily ascertaining the contents of the fluid contained within the probe. In an alternate arrangement in the prior art, a preamplifier is connected directly at the probe to amplify the signal and thereby supposedly lessen the chance of electrical interference. However, the addition of electrical components at the probe station provides a very cumbersome arrangement and makes it very difficult to use more than one probe at the probe station.
Accordingly, one object of the present invention is to provide a probe or electrode structure that is not necessarily provided with a shielding about the probe itself and that does not require the use of preamplifiers at the probe.
Another object of the present invention is to provide a probe that is constructed primarily of a transparent material such as glass so that the liquid level in the probe can be easily monitored.
It has been found in accordance with the present invention that, rather than shielding at the probe, a relatively large shield is provided which totally surrounds the probe station and also the turntable and cups and preferably all of the structure contained above the top wall of the housing. This shield is in the form of a dome which may be of cylindrical shape and may be constructed of a plastic material such as plexiglass which either has a relatively thin layer of conductive material evaporated on the inside surface thereof or has a thin membrane affixed inside thereof which membrane has a thin conductive layer deposited thereon. This conductive layer may be a gold layer but the layer is thin enough so that viewing is readily possible through the shield.
It has been further found in accordance with the invention that it is preferred that many of the electrical components contained within the housing and including also the turntable and cover external to the housing be electrically isolated from ground. In many of the prior art designs an attempt is made to ground many of these components and it has been found that this may lead to certain electrical difficulties. For example, if some of the liquid sample that is loaded into the cup is splashed onto the underside of the metal cover a liquid bridge may occur between the cover and the solution and if the cover were grounded there may be electrical interference that is created. In accordance with the teachings of the present invention, however, the cover is isolated. Futhermore, the relay device, for example, is also floating and isolated from ground.
In accordance with the present invention another important feature that has been discovered is that because of the unshielded metal especially within the housing, there may develop a static charge. To overcome this problem high quality capacitors have been used. For example, the entire cover and plenum metal assembly above the top of the chassis is connected by a high quality capacitor to ground and the relay device within the housing which is floating is also connected by a high quality capacitor to ground.
In accordance with the present invention it is also taught that the electrostatic shield in the form of the rather large dome that surrounds the turntable and cover, be movable between two positions by means of a handle on the housing. In one position of the handle the shield rests upon the top of the housing and forms an enclosed shield about the components outside of the housing. In order to load samples onto the turntable the handle is moved to a second position causing a lifting of the shield, permitting sufficient access to the turntable to permit the samples to be loaded thereon.
It has been mentioned that in accordance with the invention many of the components are maintained in a floating state. However, a ground wire is provided to the shield. This ground wire is coupled through the center support of the shield and is conductively coupled to the membrane or metallic layer within the dome of the shield.